Using a high-speed computer-controlled feedback system capable of real-time applications we will place living ventricular muscle preparations under the force/length constraints which would occur if the muscle were in the intact ventricle. Studies of anticipated increasing difficulty are planned. First, physiologically sequenced isometric and isotonic relaxation will be examined. The effects of a simulated atrial "kick" will be studied. Auxotonic phase of shortening and lengthening will be substituted for isotonic phases of contraction. Valvular stenosis and regurgitation will be modeled and the responses of normal, hypertrophied and failing cardiac muscle will be examined. As familiarity with the newly developed experimental system is optimized, an investigation of some of the more complex problems in cardiac muscle mechanics will be initiated. These will include studies of the passive elements of heart muscle, muscle models, and an investigation of the contractile element.